RFSNR Estimation Circuit for FM Receiver Audio Fidelity
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Solution Overview
Problem
Existing signal quality estimation methods in radio receivers, such as FM receivers, face challenges in accurately determining signal-to-noise ratio (SNR) due to variability in antenna impedance and noise figure, especially in portable devices, leading to suboptimal stereo noise control (SNC) mechanisms like soft mute and mono-stereo blending.
Innovation Solution
A circuit comprising a frequency shifter, filter, and RFSNR estimation circuit that converts demodulated signals to a signal-free region around DC, rejects signal components, and estimates RFSNR based on noise power in that region, enabling dynamic control of demodulated signals for improved audio fidelity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If existing signal quality estimation methods are used in portable FM receivers, then the device complexity is reduced, but the measurement precision of signal-to-noise ratio deteriorates due to variability in antenna impedance and noise figure
Solution Approach 1:
The patent introduces an intermediary signal processing path that frequency-shifts the demodulated signal to a signal-free region around DC, where only noise components exist. This intermediary measurement path allows accurate noise power estimation without being affected by the variability of antenna impedance and noise figure in the main signal path, thereby resolving the contradiction between measurement precision and device complexity
Solution Approach 2:
The patent replaces the conventional direct measurement approach (which is sensitive to hardware variations) with a signal processing-based indirect measurement approach. By frequency-shifting the signal to DC and measuring noise power in the signal-free region, the system substitutes a more precise but complex signal processing method for the simpler but less accurate direct measurement, resolving the contradiction between measurement precision and device complexity
2Measurement precision
If noise power is measured in the signal-free region around DC, then the measurement precision of RFSNR is improved, but the device complexity increases due to additional frequency shifter and filter circuits
Solution Approach 1:
The frequency shifter circuit serves multiple functions: it frequency-shifts the demodulated signal to the signal-free region for accurate noise measurement, and the same shifted signal path can be used for other processing functions. This multi-functionality reduces the overall device complexity despite adding the frequency shifter, as it eliminates the need for separate dedicated noise measurement path
Solution Approach 2:
The patent merges the noise measurement function with the existing signal processing chain by integrating the frequency shifter and filter circuits into the demodulator output path. The noise power measurement is performed as part of the existing signal processing workflow rather than as a separate independent measurement system, thereby reducing overall device complexity while maintaining high measurement precision
3Reliability
If soft mute and mono-stereo blending control is implemented, then the audio quality is improved under varying signal conditions, but the device complexity increases due to additional control mechanisms
Solution Approach 1:
The patent implements feedback control where the accurately measured RFSNR value from the signal-free region is fed back to the control circuit, which then dynamically adjusts the soft mute attenuation factor and mono-stereo blending factor. This feedback mechanism ensures optimal audio quality consistency under varying signal conditions while keeping the control circuit relatively simple by using a single primary control parameter (RFSNR) to drive both control functions
Solution Approach 2:
The patent makes the audio output characteristics dynamic by continuously adjusting the soft mute and mono-stereo blending parameters based on the real-time RFSNR measurement. The control circuit dynamically scales the stereo (L-R) signal and blends it with the mono (L+R) signal, and dynamically attenuates the final audio output at low RFSNRs, thereby maintaining optimal audio quality across varying signal conditions without requiring complex static control mechanisms
Data Source
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Figure 4A
AI summary
In an embodiment, a method of signal quality estimation and control includes generating a demodulated signal associated with a radio signal (802). A value is estimated of radio frequency signal-to-noise ratio (RFSNR) for the radio signal (806) based on information associated with a quality of the demodulated signal (804). Estimating the value of RFSNR facilitates in signal quality estimation of the radio signal and in control of the demodulated signal.